Enkephalin-like immunoreactive material present in isolated bovine chromaffin cells is released spontaneously when cells are incubated in Krebs-Ringer buffer. This release is increased by potassium and by acetylcholine stimulation. In both cases the stimulated increase in release is calcium-dependent. Preliminary evidence indicates that under these experimental conditions the response to acetylcholine appears to be mediated by nicotinic receptors. Biochemical characterization indicates that enkephalin-like material is contained in and released from the chromaffin cells as high molecular weight forms.